1. Field of the Invention
The present invention relates to an automatic pipette and more particularly an improved automatic pipette which directly detects the volume of an inserted syringe, thus eliminating the need for operator specification of this quantity.
2. Brief Description of the Prior Art
Automatic pipettes and diluters are well known in the chemical analysis art and in the medical laboratory analysis art. Automatic pipettes are used to repetitively deliver precise quantities of reagents. Where many samples must be analyzed, or where many repetitions of a single analysis must be made for statistical purposes, manual pipetting is less desirable than automatic delivery of precise reagent volumes. Automatic pipetting reduces errors associated with analyst fatigue, perception and technique. The advantages of automatic pipetting and dilution over manual methods become even more significant when micro quantities of reagents must be used in analysis.
Automatic pipettes typically employ the controlled advance of a plunger through a syringe barrel to achieve the metered delivery of fluid. In an automatic pipette the operator typically presets the amount of fluid to be delivered, and the pipette itself controls the advancement of the plunger through the syringe barrel. This may be through a stepping motor and screw mechanism and associated control means. For example, in U.S. Pat. No. 3,915,651, granted Sept. 22, 1972 to H. H. Nishi, the plunger is connected to a micrometer screw which is rotated by a stepping motor. The Nishi pipette is controlled by an electronic indexer which is present by the operator to define the number of increments through which the motor is stepped. The same function can be performed by a microprocessor.
Motor-driven automatic pipettes may deliver fluid or fluids from two or more syringes simultaneously. This may be accomplished using separate stepping motors, control circuits, et al. for each syringe. The syringes themselves may be different in size. Sequential delivery from two or more different syringes may also be accomplished. In addition, fluid may be delivered from one syringe into another partially filled with a second fluid in order to dilute the first fluid. The diluted fluid in the second syringe may subsequently be delivered to an external receptacle.
An alternative to the motor-driven automatic pipette is the manual automatic pipette. In the manual automatic pipette the operator effects filling and delivery from the pipette, typically by depressing and releasing a thumb-operable button. The pipette is automatic in the sense that the operator need not visually confirm the volume taken up and delivered by the pipette as in manual pipetting. The pipette may permit the operator to vary the stroke of the piston, in order to vary the volume delivered, as in U.S. Pat. No. 3,766,785. However, the manual type of automatic pipette is generally manufactured to repetitively deliver only a standard single volume of fluid and the volume to be delivered is not quickly altered. Because "automatic pipette" is used in the art to refer to both manual and motor-driven pipetting devices, the latter device may be referred to as a motor-driven automatic pipette.
It is advantageous that the syringe barrel and plunger combinations of different total volume displacements be available for use in an automatic pipette. This is because the precision of fluid delivery depends upon the minimum amount which the syringe piston must be displaced within the syringe barrel. This, in turn, is typically limited by the minimum increment of the control and stepping motor and the fineness of the screw thread through which the syringe piston is advanced. The volume displaced by the syringe is related to the axial displacement of the syringe piston by the cross-sectional area of the syringe barrel. By using syringe barrels of different cross-sectional area, that is, different size syringes, the minimum volume delivery increment and the precision of delivery can be altered to suit the task at hand.
Typically, a motor-driven automatic pipette is preset by the operator to the volume to be delivered. In order for the pipette to compute the plunger displacement required to achieve the desired delivery volume, the cross-sectional area of the syringe barrel which is actually in place in the pipette must be made known to the pipette. One way in which this information can be made available to the pipette is for the operator to input this information at the time a syringe barrel and plunger is fitted to the pipette. A variety of different methods may conceivably be used to transfer this information at that time. For example, a switch characteristic of syringe size may be physically set by the operator. Whatever conventional method is adopted, the fact that syringe size is selected by the analyst introduces an opportunity for operator error into the analytical task. For example, the switch, referred to above, may not be accurately set initially, possibly necessitating extensive retesting. In medical laboratory analysis especially, where sample volumes may be very small and acquired under difficult-to-reproduce conditions, it is desirable to eliminate as far as possible all sources of operator error in analytical procedures.
Thus, one of the objects of this invention is to reduce the amount of information which an operator of an automatic pipette must input when a new interchangeable syringe assembly is installed. Another object of the invention is to eliminate the possibility of operator error in inputing the volume which characterizes one of a series of interchangeable syringe assemblies for an automatic pipette when installing a new assembly in the pipette. Another object of this invention is for the pipette to automatically indicate, without operator intervention, the effective volume of one of a series of interchangeable syringe assemblies in a motor-driven automatic pipette. These and other objects of the present invention will become apparent to one skilled in the art in the following description of the invention and its preferred embodiment.